GCSE Chemistry Topic 4: Chemical Changes worksheet featuring key topics like redox reactions, titrations, and electrolysis with practice questions and exercises.

Absolutely! Let’s solve the problems from your GCSE Chemistry worksheet step by step.

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## ✔ Main Stage

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1a) Iron can displace copper from copper sulphate. Write the equation for this reaction.

This is a displacement reaction. Iron (Fe) is more reactive than copper (Cu), so it displaces copper from copper(II) sulphate solution.

Balanced chemical equation:

> Fe + CuSO₄ → FeSO₄ + Cu

✔ This shows iron replacing copper in the compound, forming iron(II) sulphate and solid copper.

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1b) Write an ionic equation for this reaction and state what is oxidised and what is reduced.

First, write the full ionic equation by splitting all soluble ionic compounds into ions:

- CuSO₄(aq) → Cu²⁺(aq) + SO₄²⁻(aq)
- FeSO₄(aq) → Fe²⁺(aq) + SO₄²⁻(aq)

So the ionic equation becomes:

> Fe(s) + Cu²⁺(aq) → Fe²⁺(aq) + Cu(s)

*(Note: SO₄²⁻ is a spectator ion — it doesn’t change — so we omit it in the net ionic equation.)*

#### 🔬 Oxidation and Reduction:

- Iron (Fe) goes from 0 oxidation state → +2 → Oxidised (loses electrons)
- Copper ion (Cu²⁺) goes from +2 → 0 → Reduced (gains electrons)

✔ So:
- Oxidised: Fe
- Reduced: Cu²⁺

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2) Write the ionic equation for the reaction below:

> Pb + CuCl₂ → PbCl₂ + Cu

Again, split soluble ionic compounds into ions:

- CuCl₂(aq) → Cu²⁺(aq) + 2Cl⁻(aq)
- PbCl₂(aq) → Pb²⁺(aq) + 2Cl⁻(aq)

Spectator ions: Cl⁻ (appears unchanged on both sides)

Net ionic equation:

> Pb(s) + Cu²⁺(aq) → Pb²⁺(aq) + Cu(s)

✔ This shows lead displacing copper ions — lead is oxidised, copper ions are reduced.

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For the following redox reactions, write half equations...

We need to write reduction half-equations (as per column heading “Reduction”).

Remember: Reduction = gain of electrons

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#### 1. CuO + Mg → MgO + Cu

Focus on copper: Cu²⁺ in CuO → Cu (0)

> Cu²⁺ + 2e⁻ → Cu

✔ Reduction half-equation

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#### 2. Fe₂O₃ + 2Al → 2Fe + Al₂O₃

Focus on iron: Fe³⁺ in Fe₂O₃ → Fe (0)

Each Fe³⁺ gains 3 electrons → Fe

> Fe³⁺ + 3e⁻ → Fe

✔ Reduction half-equation

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#### 3. CuSO₄ + Zn → ZnSO₄ + Cu

Focus on copper: Cu²⁺ → Cu

> Cu²⁺ + 2e⁻ → Cu

✔ Same as before!

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#### 4. 2KBr + Cl₂ → 2KCl + Br₂

Focus on chlorine: Cl₂ → Cl⁻

Each Cl atom gains 1 electron → Cl⁻

> Cl₂ + 2e⁻ → 2Cl⁻

✔ Reduction half-equation

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#### 5. 2NaCl + F₂ → Cl₂ + 2NaF

Focus on fluorine: F₂ → F⁻

> F₂ + 2e⁻ → 2F⁻

✔ Reduction half-equation

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## ✔ Boost Section

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1) Write half equations for the reactions below.

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#### Reaction 1: H₂S + Cl₂ → S + 2HCl

This is a redox reaction.

- Sulphur in H₂S goes from -2 → 0 → Oxidised
- Chlorine in Cl₂ goes from 0 → -1 → Reduced

##### ➤ Oxidation half-equation (S²⁻ → S):

> H₂S → S + 2H⁺ + 2e⁻

*(Note: We include H⁺ because it’s in acidic medium — common in these contexts.)*

##### ➤ Reduction half-equation (Cl₂ → Cl⁻):

> Cl₂ + 2e⁻ → 2Cl⁻

✔ So you’d write:

| Oxidation | Reduction |
|-----------|---------|
| H₂S → S + 2H⁺ + 2e⁻ | Cl₂ + 2e⁻ → 2Cl⁻ |

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#### Reaction 2: Zn + 2HCl → ZnCl₂ + H₂

- Zinc goes from 0 → +2 → Oxidised
- Hydrogen in H⁺ goes from +1 → 0 → Reduced

##### ➤ Oxidation half-equation:

> Zn → Zn²⁺ + 2e⁻

##### ➤ Reduction half-equation:

> 2H⁺ + 2e⁻ → H₂

✔ So you’d write:

| Oxidation | Reduction |
|-----------|---------|
| Zn → Zn²⁺ + 2e⁻ | 2H⁺ + 2e⁻ → H₂ |

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2) A scientist tries to react sodium chloride with bromine under normal conditions. Explain why this reaction did not work.

This is about reactivity of halogens.

Bromine (Br₂) is less reactive than chlorine (Cl₂). In displacement reactions, a more reactive halogen can displace a less reactive one from its salt.

Here, the scientist is trying:

> 2NaCl + Br₂ → ?

But bromine cannot displace chloride ions because chlorine is higher in the reactivity series of halogens.

✔ Explanation:

> Bromine is less reactive than chlorine. Therefore, it cannot displace chloride ions from sodium chloride. Only a more reactive halogen (like fluorine or chlorine) can displace a less reactive one (like bromine or iodine) from its salt.

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## 🎯 Final Summary:

You’ve now completed all parts of the worksheet! Here's a quick recap:

- Displacement reactions follow reactivity series.
- Ionic equations show only species that change.
- Redox involves oxidation (loss of e⁻) and reduction (gain of e⁻).
- Halogen reactivity decreases down the group: F > Cl > Br > I.

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